A general statistical theory is developed for the masking effect of reverberation on the intelligibility of words. Speech is considered a series of discrete pulses distributed statistically over a 30-db range in sound pressure level in a given frequency band. The articulation index is calculated as a function of reverberation time, using preliminary values of speech pulse lengths and spacings obtained from Visible Speech spectrograms. The percent articulation for words is then calculated from the articulation index and is compared with Knudsen's experimental values. The theoretical values agree precisely with the measured values at reverberation times less than two seconds and differ by less than 17 percent out to six seconds. The calculations are extended to include a combination of background noise and reverberation.

No abstract

Rev. Mod. Phys. 16, 69 (1944) j~D D to the last paragraph of Section 20 the following sentences and equation: For instance, f' or wave-lengths long compared to the thickness of material plus air backing, the wall impedance is approximately that of two equivalent circuit arms in parallel, the impedance of the two arms being r"L+(ipc'/MPL) and r"L+(i pc'/coL, B, ) Transverse waves suppressed .where 8 cos yl, Transverse waves allowed This corresponds approximately to the circuit in Fig. 8a. In the present instance it is somewhat more accurate to insert a resistance in both arms and to neglect the inductance. The third arm, corresponding to panel motion, will be discussed in the next section. Erratum: Surface Roughness and Sliding Friction J. J. BIKKRMAN [Rev. Mod. Phys. 16, 53 (1944)g W~N page 63, in the second paragraph in the left-hand column, (Section 1I, 6) should be (Sectioñ JJ, 2, b3).

This paper extends and unifies the analysis of sound absorptive structures consisting of (1) a layer of porous material separated from (2) a rigid wall by (3) an air cavity, which may or may not contain (4) separating partitions; and the porous material may or may not be covered with (5) a perforated facing. The normal impedance of the porous layer is expressed in a series form in which the first term suffices for a very thin layer. Equations and design charts are given for the impedance and the absorption coefficient, as functions of angle of incidence, for the complete structure with and without partitions. The statistical coefficient, averaged over angle, is plotted for each case over the commonly encountered ranges of all design variables. Calculated coefficients for eight different designs are compared with those measured by the reverberation method and found to be in good agreement.

An approximate analysis of the acoustic behavior of perforated facings for absorptive materials is outlined and is reduced to a design chart. Only the mass reactances of the holes and of the facing material are included in the analysis. This simplified picture is adequate for many practical problems, but breaks down for certain conditions which are discussed. If the acoustic impedance of the unfaced material is known, absorption coefficients for the material with perforated facing can be read directly from the chart.